The invention relates to the processing of crack-free ceramic matrix composites.
This process represents a major improvement to the state-of-the-art ceramic composite fabrication process. (Ceramic composite or ceramic matrix composite (CMC) as used herein, refers to continuous ceramic fibers reinforcing a ceramic “matrix” where the matrix is the material around the fibers bonding the assemblage into a rigid body.) Current oxide-oxide composite matrices suffer from transverse and interlaminar matrix cracks due to drying and sintering shrinkage in the presence of the constraining force imposed by a rigid and dense fiber network.
Currently, oxide-oxide ceramic matrix composites are produced by infiltrating oxide fibers, woven fabric or fiber preforms (fibers woven into a multi-axial structure) with an aqueous ceramic slurry. This slurry consists of ceramic powder, water, and various additives to improve the dispersion of the powder. The infiltrated fiber (woven fabric for example) is then stacked into a desired thickness and orientation, followed by consolidation using pressure and warm temperatures (generally below 400° C.). This step drives off the water and some other volatiles, yielding a solid but very weak “green” body. The green body is then sintered at high temperature (1100° C. or above) in air to produce the final ceramic composite. The sintering temperature is generally limited to a maximum of ˜1250° C. due to strength loss in the fiber that results at higher temperatures. Due to the nature of the process, only porous matrix composites (>20% composite porosity) are produced using this method. Further, transverse and interlaminar cracks occur to relieve stresses that occur during drying and subsequent sintering of the matrix. The porosity and cracks are defects that limit the mechanical properties of the composite.
The processing method discussed in the current invention provides a method to avoid both the drying and sintering shrinkage cracks, through a combination of freeze forming and the use of a non-shrinking matrix.
Published work has demonstrated the efficacy of using camphene as an alternative vehicle to water for freeze-casting of monolithic (non-fiber-reinforced) ceramic. See, for example, K. Araki and J. W. Halloran, “New Freeze-Casting Technique for Ceramics with Sublimable Vehicles,” J. Am. Ceram. Soc., 87 [10] 1859-1863 (2004).
This invention utilizes camphene-based freeze casting for solving a very difficult processing problem; namely, the fabrication of continuous-fiber reinforced ceramic composites.